ABSTRACT: Cyanophage PP (isolated from Plectonema boryanum and Phormidium foveolarum and named after the respective first letters of its 2 hosts) is a short-tailed, icosahedral-shaped, and double-stranded DNA virus and can be frequently detected with a high abundance and activity in many eutrophic lakes in Wuhan City, PR China. To understand how the virus survives solar UV-B damage, we examined the decay of cyanophage PP induced by solar UV-B radiation and the photoreactivation repair by host cyanobacteria, on 1 d in each of the 4 different seasons throughout a year. The UV-B transparent or non-transparent bag containing the cyanophage PP was exposed to sunlight at different water depths and was collected at varying time points. The collected sample was treated with or without radiation of a photoreactivating wavelength (300 to 500 nm). The decay ratio (here designated as the percentage of infectivity lost) for cyanophage PP from all samples caused by UV-B radiation ranged from 29.8 to 92.1%. Season and water depth were the main factors influencing the decay ratio, while long exposure time had less effect. The repair ratio (here designated as the percentage of revived infectivity by photoreactivation) for cyanophage PP reached a maximum during the first 1 to 3 h of exposure, at all depths. The maximum repair ratios of photoreactivation for each of the decayed samples, ranging from 7 to 59%, were negatively correlated with the decay ratio. In most cases, the repair ratio of photoreactivation reached a maximum in the first 4 h. Our results indicate that solar UV-B radiation may decrease the infectivity of cyanophage PP in shallow freshwater bodies in an extraordinarily fast and effective manner, whereas rapid photoreactivation may contribute to balancing the rapid phototoxicity.